of the Seismological Society of America, Vol. 77, No. 5, pp. 1704-1717 - PDF document

of the Seismological Society of America, Vol. 77, No. 5, pp. 1704-1717, October 1987 OF FIRST-ORDER MANTLE REVERBERATIONS JUSTIN REVENAUGH AND THOMAS H. JORDAN have observed first-order mantle reverberations, specifically SH-polarized reflected at near-normal incidence from upper mantle discontinuities, as discrete phases on long-period digital seismograms of the SRO networks. Such arrivals correspond to OF FIRST-ORDER MANTLE REVERBERATIONS SCS2S650+S 2. ray paths for three members of the family $650+S}, first-order rever- beration from the topside of the 650 km discontinuity, for a surface-focus source. The three phases shown here are dynamic analogs; i.e., they have the same travel time, amplitude, and phase at the receiver and thus constructively interfere to produce an observable arrival. 1 PARAMETERS OF THE EVENTS USED IN THIS STUDY Time Latitude Longitude Depth A to KIP Date (UTC) (deg.) (deg.) (km) 21 July 1973 04:19:13.7 24.83S 179.19W 419 5.9 50.37 2 19Dec. 1973 12:55:51.1 20.60S 176.32W 229 5.8 45.43 3 28Dec. 1973 05:31:03.8 23.88S 180.00W 526 6.4 49.85 4 23Mar. 1974 14:28:33.0 23.92S 179.88E 515 6.1 49.94 5 4June 1974 04:14:13.8 15.89S 175.04W 275 6.0 40.67 6 21 Oct. 1974 04:12:28.7 17.97S 178.49W 596 6.0 43.99 7 22 Feb. 1975 22:04:33.5 24.98S 178.88W 373 6.2 50.38 8 19 Nov. 1975 06:18:33.7 24.01S 179.09E 551 5.8 50.35 9 3Feb. 1976 12:27:30.1 25.14S 179.69E 477 5.8 51.10 10 21 Jan. 1977 06:11:05.3 !8.06S 178.37W 602 5.8 44.01 11 6 July 1977 11:28:31.5 21.00S 178.58W 597 5.8 46.70 through the transition zone, the average shear attenuation of the upper and lower mantles, and the reflection coefficient of the 650 km discontinuity. concentrated our initial efforts on paths within the Tonga-Hawaii corridor, which has a favorable geometry for the observation of mantle reverberations. The properties of multiple along this relatively uniform swath of old oceanic lithosphere have been investigated extensively by Jordan and Sipkin (1977) and Sipkin and Jordan (1979, 1980a, b). Seismograms from the HGLP instruments at station KIP, on the island of Oahu, were provided to the authors by S. Sipkin of the U.S. Geological Survey. Eleven Tonga-Fiji events with focal depths ranging from 229 to 602 km were selected for detailed analysis; their source parameters are listed in Table 1. The events had simple time functions, good ratios at KIP, and depths of focus large enough to separate the At these epicentral distances (40 ° to 51°), the reverberative interval is about 50 min long and includes reverberations with reflection numbers n between two and four (Figure 1). Data 400- + 650- t 4~0 5"0 6"0 7~0 Time (min) 4. SH-polarized seismograms showing the reverberative interval for event 4 (h = 515 km, A = 49.9°). trace data; all others are synthetics computed by geometrical optics ray theory. The trace labeled the beginning with traces labeled +, 400 +, 650- 400- are the first-order reverberations reflecting from the top and bottom of the 650 and 400 km discontinuities, respectively. The trace labeled 2nd contains all the second-order reverberations from the 400 and 650 km discontinuities. The first-order reverberations decay slowly with increasing multiple number n, owing to the constructive interference among members of each dynamic ray family. Time is minutes after origin time. 410 5'0 6'0 7'0 80 (rain) 5. SH-polarized seismograms showing the reverberative interval for event 2 (h = 229 km, A = 45.4"). trace data; all others are synthetics computed by geometrical optics ray theory. Conventions are the same as in Figure 4. OF FIRST-ORDER MANTLE REVERBERATIONS ~--|-- l--lt--r--l--l--~ ..... 1 I I I I I ,.-, -0.2 o -0.6 190+60 -0.8 I I I I t 5 10 15 20 25 30 35 40 1.0 I o.5 o.o ................ }__ -0.5 ~- -1.0 I I 5 10 15 20 25 30 55 40 Frequency (mHz) 9. Modulus and phase of the lower mantle differential response operator, showing estimates derived from 18 $650-S} - sScS, pairs using the phase equalization and stacking algorithm of Jordan and Sipkin (1977). Lines represent predicted differential response for a linear, causal, constant-Q model with QLM ---- 190. Error bars are la. 8.4 0 - 8.2 x o8.1 8.0 7.8 7.7 OBSERVATIONS OF FIRST-ORDER MANTLE REVERBERATIONS 1715 QLM 1 O0 gO 80 70 275 250 225 200 I 12 I 1. 114 Q-!UM (xlO 2) 8.4 8.5 o8.1 7.8 0 0.50 Q-lud (xlO z) 0.50 0.45 0.40 0.35 O0 90 80 I t I f 0 1.2 1.4 (xlO 2)